The concept of a step-up coil mounted directly on each spark plug of an internal combustion engine is over 50 years old. See, for example, U.S. Pat. Nos. 1,624,951; 1,722,269; 2,033,745; 2,180,358; 2,266,614; 2,414,692; 2,446,888; 2,455,960; 2,459,856; 2,467,531; 2,467,725; 2,482,884; 2,635,202; 3,716,038; 3,935,852; and 4,090,125. The major advantages of such an ignition system are (a) elimination of exposed high tension leads, (b) minimization of the length of the high tension leads thereby reducing losses due to the leakage current and capacitive loading, and (c) inherent safety resulting from the impossibility of disconnecting the coil from the plug with the primary connection plugged in.
However, despite these advantages the use of individual step-up coils with each spark plug of an internal combustion engine has not seen widespread use. One substantial reason has related to heating of the coil. The proximity of the coil (and the insulating materials associated therewith) to the spark plug results in the coil and the insulating materials being subjected to heat coming off of the engine. Modern materials go some distance to overcome this disadvantage. Even so, two other substantial problems remain with integral coils. Breakdown of the spark plug seals allows combustion gases in the engine cylinder to eventually leak to the termination area between the plug and the coil. This creates very high temperatures and pressures which can destroy the coil electrically or even cause mechanical disintegration. In either event, the operator must replace not only the spark plug but the coil. Secondly, there has been a lack of a simple way of verifying that the spark plug is firing when the coil surrounds the terminal atop the spark plug. A common method used with unshielded systems is a neon bulb indicator (see for example, Peters U.S. Pat. No. 2,181,149 and U.S. Pat. No. 2,245,604).
According to this invention there is provided an integral step-up coil spark plug for an internal combustion engine which overcomes the problems of spark plug leakage and enables easy verification that the spark plug is firing.